Intravascular ionizing radiation therapy is being used increasingly to treat vascular disease, and has been proposed as both a primary and a secondary therapy for treating vascular restrictions. Clinical studies have shown that ionizing radiation may be effectively used to inhibit or prevent restenosis after percutaneous translumenal coronary angioplasty (PCTA). For example, U.S. Pat. No. 5,643,171 to Bradshaw et al disclose a method and apparatus for intravascular radiotherapy for prevention of restenosis following angioplasty or other procedures that cause smooth cell proliferation.
As best seen in FIG. 1 of Bradshaw et al., a catheter 10 is illustrated having an elongate shaft 12 with a distal treatment section 14 and a distal tip 16. Attached to the distal treatment section 14 is a centering balloon 40. The elongate shaft 12 also includes a treatment channel 20 as best seen in FIG. 2. The treatment channel 20 allows for the introduction of a source wire (not shown) having a distal radioactive section. With this design, the catheter 10 allegedly maintains the treatment channel 20, and thus the source wire, in the center of the vessel, despite vessel curvature in the region the vessel being treated, for uniform delivery of radiation.
One disadvantage of this particular design is the arrangement of the catheter 10 relative to the guide wire 32, which may block radiation from reaching the vessel wall 30. In particular, the shaft 12 includes a distal Monorail.RTM.-type guide wire lumen 24 that allows the catheter to be advanced over the guide wire 32 until the treatment section 14 is disposed in the target area 34 of the blood vessel 30. The distal Monorail.RTM.-type lumen 24 opens at the distal tip of the shaft and exits through the lateral surface of the shaft 12 distal of the balloon 40. Thus, the guide wire 32 extends adjacent the catheter 10 and centering balloon 40 at the target area 34 of the blood vessel 30.
Because guide wires are conventionally formed of metal alloys such as stainless steel, the guide wire 32 will tend to attenuate radiation emitted by the source wire disposed in the treatment channel 20. Attenuation of the radiation causes a shadow to be cast on the vessel wall 30 in the target area 34 such that a portion of the target area 34 is not uniformly exposed to ionizing radiation. Failure to expose the entire target area 34 to ionizing radiation may give rise to restenosis at the unexposed or underexposed region. The recurrence of restenosis anywhere in the target area 34 is clearly disadvantageous since the primary objective of the therapy is to prevent or otherwise inhibit restenosis.
The initial response to solving this problem may be to move (e.g., retract or withdraw) the guide wire 32 to avoid blocking radiation. However, retraction of guide wire in the proximal direction such that the guide wire 32 does not extend across the target area 34, is not a particularly viable option because vascular access across the target area 34 would be lost and access to the guide wire lumen 24 at the distal tip of the shaft 12 would also be lost. In many instances, it is undesirable to lose vascular access across the target area 34 since the restriction may recoil rendering it difficult if not impossible to renavigate the guide wire 32 across the target site 34. Without the guide wire 32 disposed across the target area 34, it would be difficult to redilate or otherwise treat the vascular restriction. In addition, losing access to the guide wire lumen 24 makes it difficult, if not impossible, to steer or guide the catheter 10 through the vascular channel. Thus, it is extremely undesirable to retract the guide wire 32 in the proximal direction. Because it is undesirable to retract the guide wire 32 in the proximal direction, the guide wire 32 must be left in place where it will inevitably attenuate radiation emitted from the source wire .